"This could make NOAA weather radio practically obsolete."
While cell phone alerts are certainly reaching many more people who do not have a weather radio, I don't think they will make weather radio obsolete for some time, if ever. If your cell phone battery dies, or you forgot your phone, a weather radio on premise that you don't have to think about is much more useful. In a power outage you can still use off the shelf batteries to power a weather radio, where you would have a much harder time continuously powering a cell phone. Even more importantly, in many rural and mountainous areas, weather radio reception is MUCH more trustworthy due to the frequencies of transmission. Thats not even to mention that CMAS/WEA alerts are not provided by every carrier or in every region yet.
Its cool that they have these cell phone warnings but I don't think its safe to rely on them alone just yet.
As far as where the alerts are sent, from NOAA's page on WEA: "WEA messages are broadcast using radio-like technology from cell towers in, and sometimes around, the actual warning area. Therefore, an alert can reach cell phones outside of the actual warning area depending on the broadcast range of the cell towers which broadcast the alert. This overreach is typically more prevalent in rural areas than in more densely populated cities." So I think the answer to whether its polygons or counties is: yes, and no - its an approximation based on the warning area.